This invention relates to an antenna for motor vehicles, and more specifically to a dipole antenna for motor vehicles.
Antenna technology in motor vehicles become increasing important with the recent rise in mobile communications and convenience items, such as passive entry systems. These systems commonly employ radio waves and, thus require an antenna both for receiving and transmitting. The nature of motor vehicles creates some rather unique problems in antenna design. The antenna must be small and have a low profile, yet be capable of providing a sufficiently wide frequency band and be easily tunable to a desired frequency.
Passive entry systems present additional problems. In passive entry systems an antenna having specific directionality, i.e. separation of one side of the vehicle from the other, is preferred. Further, an antenna having a substantially smooth radiation pattern is preferred. An antenna without a smooth radiation pattern will have significant gaps or voids in the directional pattern from which the antenna may receive signals. In other words, the antenna will not be able to receive a signal generated from the direction corresponding to a gap or void in the radiation pattern and the system will not operate.
U.S. Pat. No. 4,914,447 discloses an antenna for mobile telephone communications. The antenna is mounted on a glass panel of an automobile. The antenna comprises a horizontal element and two vertical elements extending from each end of the horizontal element. The first feeding point is attached to the end of one of the vertical elements. A second feeding point is connected to an earth-line, i.e. ground, such that the antenna in the '447 patent is a monopole antenna.
However, monopole antennas are not completely satisfactory in mobile vehicle applications. As the monopole is grounded to its surroundings, it is more difficult to control impedance or VSWR. It is desirable to tune impedance to match the impedance of a receiver to maximized power transfer. Monopole antennas make such tuning more difficult. Furthermore, monopole antennas do not provide for a substantially smooth radiation pattern as desired in passive entry systems.
U.S. Pat. Nos. 4,907,006, 4,968,984 and 5,146,232 all disclose antennas mounted on vehicles for mobile communications. All three patents disclose cavity-type antennas, which employ sheets of metal bent and fed in different manners. While some embodiments of the '984 patent have an "F" shaped profile, all three patents disclose monopole antennas.
U.S. Pat. Nos. 4,868,915, 4,873,530, and 4,942,393 all disclose passive entry systems. All three systems employ loop or coil type antennas. In the cases of the '915 patent and '393 patent multiple coil antennas are employed. However, loop or coil antennas actually increase the number of voids or nulls in the radiation pattern rather than smoothing it. Furthermore, multiple antennas require additional power to radiate and are more difficult to engineer into the vehicle's structure.
Accordingly, the need remains for an antenna for a motor vehicle which provides a substantially smooth radiation pattern, simplified impedance matching and bandwidth control, for convenient resonant frequency selection for the antenna and broad bandwidth potential.